1. Field of the Invention
The claimed invention relates to a down converter, and more particularly discloses a down converter having the function of adjusting filtering effect.
2. Description of the Prior Art
Please refer to FIG. 1, which is a diagram of the prior art down converter. The prior art down converter 1 comprises a housing 30, a down conversion circuit 200, and a shielding structure 10. The down conversion circuit 200 is disposed in the housing 30. The shielding structure 10 is disposed above the down conversion circuit 200.
Please refer to FIG. 2, which is a diagram of the prior art down conversion circuit 200. The down conversion circuit 200 comprises a radio frequency circuit 250 and an intermediate frequency circuit 260. The radio frequency circuit 250 is coupled with the intermediate frequency circuit 260. The radio frequency circuit 250 is able to receive multiple radio signals (for example, left-hand circular polarization signals and right-hand circular polarization signals in the same frequency band), down-convert the radio signals, and transmit them to the intermediate frequency circuit 260. The radio frequency circuit 250 comprises four antennas 211, 221, 231, and 241, a low noise amplifier 212, a radio frequency bandpass filter 214, two mixers 216 and 217, a local oscillator 218, eight intermediate frequency filters 219, 210, 222, 223, 232, 233, 242, and 243, and an intermediate frequency amplifier 213. The radio signals are received and down-converted by the elements of the radio frequency circuit 250. Moreover, the function of the radio frequency bandpass filter 214 is filtering noise of particular frequency bands (for example, from 17.3 GHz to 17.8 GHz) in the radio signals so that the mixing effect of mixers 216 and 217 can be improved.
Please refer to FIG. 1 again. Since the signals generated by the elements of the down conversion circuit 200 are transmitted by circuit conduction and radiation respectively, the shielding structure 10 is disposed above the down conversion circuit 200 in order to decrease the signal interruptions between the elements of the down conversion circuit 200. For example, please refer to FIG. 3. Multiple cavities are disposed in the shielding structure 10, wherein the multiple cavities correspond to the elements of the down conversion circuit 200 respectively. Therefore the elements of the down conversion circuit 200 can be separated from each other to decrease signal interruptions.
As limited by the precision of printed circuit fabrication process, there are unpredictable dimensional tolerances in the coupling wires of printed circuits of radio frequency bandpass filters used in down conversion circuits. Therefore the filtering effect of the filter is difficult to handle accurately. At the same time, a cavity resonance effect is also generated between the radio frequency bandpass filter and the cavities of the shielding structure. The cavity resonance effect can be as described in IEEE, Transactions on Microwave Theory and Techniques, Vol. 48, No. 8, August 2000. If there are unpredictable dimensional tolerances in the process precision of the shielding structure, the cavity resonance effect can also influence the filtering effect of the radio frequency bandpass filter.